1. Field of Invention
The present invention relates to a band-gap reference circuit, and more particularly, to a low supply voltage band-gap reference circuit.
2. Description of Related Art
Generally, in many ultra-large integrated circuit (IC) systems, basic and essential semiconductor band-gap circuits are built-in. Responsible for generating source reference current (or voltage), the band-gap circuit determines the accuracy of the whole system.
FIG. 1A is a circuit diagram of a conventional band-gap reference circuit. FIG. 1B shows a relationship diagram between the output voltage and temperature of the conventional band-gap reference circuit shown in FIG. 1A. Referring to both FIG. 1A and FIG. 1B, if the currents passing through diodes D1 and D2 are both I, the proportion between element areas of the diodes D1 and D2 is 1:n, and the resistance R1=R3, the output voltage of the operational amplifier
                              110          ⁢                                          ⁢                      V            BG                          =                ⁢                              V                          BE              ⁢                                                          ⁢              1                                +                                                    R                1                                            R                2                                      ⁢                          (                                                V                                      BE                    ⁢                                                                                  ⁢                    1                                                  -                                  V                                      BE                    ⁢                                                                                  ⁢                    2                                                              )                                                              =                ⁢                              V                          BE              ⁢                                                          ⁢              1                                +                                                    R                1                                            R                2                                      ⁢                          (                              Δ                ⁢                                                                  ⁢                                  V                  BE                                            )                                                              =                ⁢                              V                          BE              ⁢                                                          ⁢              1                                +                                                    R                1                                            R                2                                      ⁢                          (                                                V                  T                                ·                                  ln                  ⁡                                      (                    n                    )                                                              )                                                              =                ⁢                              V                          BE              ⁢                                                          ⁢              1                                +                      K            ·                          V              T                                                              ≈                ⁢                  1.205          ⁢                                          ⁢                      V            .                              Through properly adjusting the resistance ratio between R1 and R2 and the proportion between the element areas of the diodes D1 and D2 (i.e., determining the proportion n of the current density between the two diodes D1 and D2), the output voltage VBG maintains a constant value without being influenced by the temperature.
At present, considering the case of low voltage and low power, many systems of supply voltage lower than 1.2 V generally require a low supply voltage band-gap reference circuit. The conventional band-gap reference circuit shown in FIG. 1A cannot generate a low band-gap voltage for low-voltage systems. To meet the requirements of low voltage and low power, U.S. Pat. No. 6,052,020 discloses a band-gap reference circuit for generating a low output voltage through the voltage average technique. Referring to FIG. 2A, the band-gap reference circuit can generate a low band-gap voltage lower than 1.205V. FIG. 2B is a relationship diagram between the output voltage and temperature of the conventional band-gap reference circuit shown in FIG. 2A. Referring to both FIGS. 2A and 2B, the band-gap reference circuit utilizes the conventional positive temperature coefficient current generation unit 210 to provide the internal negative temperature coefficient voltage VBE and the positive temperature coefficient voltage K·VT. Next, the voltages VBE and K·VT are averaged by the voltage averaging circuit 220, and the band-gap voltage VBG lower than 1V is output. The conventional band-gap reference circuit generates the desired output voltage level by adjusting the proportion between the resistances R1 and R2.
Since more than three operational amplifiers and resistors R1, R2, R3A, and R3B of high resistance are required in the conventional art, the complexity of the band-gap reference circuit in FIG. 2A is significantly increased. Particularly, the operational amplifiers actually have different offset voltages VOS, therefore the more operational amplifiers are used, the more offset voltages VOS are generated to influence the accuracy of the band-gap reference circuit. In addition, considering the low statistic current, the resistances of resistors R3A and R3B must be set to be relatively high (e.g., 96 KΩ). The resistors R1, R2, R3A, and R3B with higher resistance require relatively large areas, thus increasing the chip area and chip cost.